It is known that fibre-reinforced elastomer gaskets are susceptible to irreversible aging. Previously, this was concealed by way of a very high asbestos content of up to 80% by weight, so that the material retained its stability despite aged elastomer. Once the health risk caused by asbestos had been recognized, attempts were made to replace it with other fibres, in particular synthetic fibres, such as p-aramid fibres, glass fibres or carbon fibres. However, the high costs for these fibres and, respectively, production problems with the use of certain fibres meant that their content had to be markedly reduced and substituted by low-cost pulverulent fillers, such as kaolins, silicon dioxide, graphite, etc.
The result of this is that the elastomeric binder then has to take on an increased proportion of the stabilization of the gasket material, although the elastomer firstly has the lowest thermal stability of the raw materials used and secondly is susceptible to aging, which impairs the long-term integrity of the fibrous and pulverulent fillers.
Depending on the R rubber type usually used as elastomer material (diene elastomers, such as NBR, SBR, NR, etc.) and the amount of the terpolymers (such as acrylonitrile or styrene) copolymerized therein, the rubber types mainly used for gasket material contain up to 35% of unsaturated carbon double bonds, which make them susceptible to oxidation, because these double bonds can react directly with oxygen to give hydroperoxides. Subsequent abstraction of the H atom on the resultant hydroperoxide leads to formation of polymer free radicals and thus to breakage of the macromolecule. The measurable effects of this mostly aerobic thermal aging include a significant increase in the modulus of elasticity and the associated reduction in flexibility, i.e. an increase in brittleness. The latter can then cause microcracking, the results of which can extend to total blow-out of the gasket, even in the case of minimal load changes at a gasketed flange, for example initiated by the dilatation of pipeline systems during the start-up and shut-down of installations.
A further problem here is also caused by the post-vulcanization of unsaturated carbon double bonds in the main chain of the elastomer material. As long as uncrosslinked dienes remain in the main chain in the presence of crosslinking reagents which have not yet been consumed (such as sulphur, sulphur donors or peroxidically generated free radicals), the elastomer material remains susceptible to post-crosslinking long after its process-vulcanization. In the case of gaskets, this likewise leads to hardening of the material, with the effects listed above.